Changes in water extractable organic matter (WEOM) in a calcareous soil under field conditions with time and soil depth

Abstract

Water extractable organic matter (WEOM) is attributed a key role in soil major biogeochemical processes. Yet, information concerning its dynamics within the soil profile and under uncontrolled field conditions is scarce, particularly in agricultural soils. Here, we present a study of WEOM dynamics across a soil profile of 100 cm over a period of seven months. Soil samples were collected from a field in the French Mediterranean region. Prior to sampling, maize residues were incorporated into soil down to around 20 cm. WEOM was fractionated on XAD-8/4 resins connected in tandem and analysed using UV/Vis absorbance and fluorescence spectroscopy. The overall data were further investigated by principal components analysis (PCA). The average concentrations of bulk WEOM decreased significantly with increasing soil depth as a result of the depletion of all its subsequent hydrophobic (HPOA), transphilic (TPIA), and hydrophilic (HPIA) acids fractions. In deeper soil layers, WEOM was most severely depleted in HPOA leading to significant decrease in its aromaticity estimated by UV specific absorbance at 254 nm (UVSA 254). Typical three-dimensional fluorescence spectra of WEOM exhibited two humic-like peaks, A and C, persistent across the entire soil profile. A low-intensity shoulder, H, attributed to lignin oxidative degradation by-products was also perceptible in WEOM from the soil surface layer. The absence of fluorescence signals attributed to labile and protein-like structures suggested highly altered status of WEOM regardless of soil depth. This is most probably related to Ca ++ preference to form links with microbial degradation by-products leading to intensive degradation of labile structures. Further alteration of WEOM is revealed by its humic-like peaks' blue-shifting and increasing the ratio between their emission intensities, IA/IC. This indicated WEOM's enrichment in simpler, less conjugated, and older structures with depth. WEOM's alteration across the soil profile may be a consequence of low vertical export of plant residues and more significant contribution of native SOM in its production. PCA revealed the association of bulk WEOM and its subsequent fractions concentrations to the 1st factor in variables' loads suggesting significant variations over the sampling period. These variations were mainly associated to rainfall and soil moisture in the upper soil layer. The contribution of these variables in explaining the overall variance decreased gradually with depth and showed weak correlation with the distribution of bulk WEOM. Temperature was associated to the 2nd and 3rd factors in variables' loads in the three soil layers indicating limited control on WEOM dynamics. PCA revealed higher sensitivity of IA/IC to climatic variations compared to UVSA 254 suggesting its higher potential as a tracer of temporal WEOM dynamics.